Many engines are equipped with a mechanical supercharger for supercharging intake air introduced into a combustion chamber or chambers utilizing exhaust gas energy.
This type of mechanical supercharger is also known as a turbocharger. A turbocharger includes a compressor disposed at an intermediate portion of the intake passage of an engine, and a turbine disposed at an intermediate portion of the exhaust passage, and configured such that the compressor is activated due to the rotation of the turbine by exhaust gas flowing through the exhaust passage, to increase the amount of intake air introduced into the combustion chamber(s), thereby increasing the engine torque.
Besides superchargers utilizing exhaust gas energy, various types of electric superchargers, which include a compressor and an electric motor for driving the compressor, have been proposed in recent years. Such an electric supercharger is advantageous in that it is possible to arbitrarily supercharge intake air by supplying electric power irrespective of the operating state of the engine (see, for example, the below-identified Patent Document 1).
A mechanical supercharger that utilizes exhaust gas energy includes a wastegate valve for adjusting the amount of exhaust gases into the turbine by partially diverting exhaust gases. By adjusting the amount of exhaust gases that pass through the turbine with the wastegate valve, it is possible to control the supercharging pressure of intake air.
Older wastegate valves were controlled by pneumatic actuators driven by the supercharging pressure. In recent years, electronically controlled wastegate valves which are selectively opened and closed by an electric motor are also used. By electronically controlling the wastegate valve, it can be controlled even while the supercharging pressure is low, and it can also be more precisely controlled.
Since large electric power is required to drive an electric supercharger, it is preferable to power such an electric supercharger using regenerative power generated while the vehicle on which the engine having the supercharger is mounted is decelerating.
The below-identified Patent Document 2 discloses a technology for increasing the amount of regenerated energy.
That is, in Patent Document 2, the actuation of a wastegate valve is controlled according to the amount of regenerated energy generated while the vehicle is decelerating. In particular, while the vehicle is decelerating and the remaining amount of charge of the battery is low, the alternator as a dynamo-electric machine regenerates electricity. This opens the wastegate valve, thus reducing the amount of exhaust gases introduced into the turbine, so that it is possible to reduce the pressure of the exhaust gases discharged from the combustion chamber(s) into the exhaust passage.
The reduced exhaust gas pressure results in reduced pumping loss, which in turn results in reduced engine braking power due to reduced revolving resistance of the engine. This makes it possible to efficiently utilize the kinetic energy of the vehicle for the regeneration of electricity.